1. Field of the Invention
The present invention relates to glass laminates. The present invention particularly relates to laminates of glass and polyvinylbutyral, and a process of preparing same.
2. Description of Related Art
Glass laminates that include plasticized polyvinyl butyral (PVB) interlayers can be used in various applications, including use in automotive safety glass applications such as windshields and side glass; in architectural applications such as windows, doors and/or building panels; and in various other applications such as in display cases, as shelving, and the like.
Glass/PVB laminates can be prepared by conventional methods. Typically, a laminate can be prepared by first positioning a sheet of PVB between two pieces of glass to obtain an assembly, and trimming the excess PVB interlayer. A “pre-press” is obtained from the assembly by removing air trapped between the glass and the interlayer, and then sealing the edges. A conventional method for edge sealing requires placing the assembly inside of a rubber bag and removing the air from the bag by applying vacuum. The rubber bag and contents can then be passed through a furnace wherein the temperature is increased to about 135° C. in order to obtain the pre-press. A pre-press so obtained can be heated in an autoclave wherein heat and pressure are applied, residual air is dissolved in the PVB interlayer, and bonding occurs between the interlayer and the surface being laminated.
An interlayer having a smooth surface can present problems during the assembly and de-airing steps of a lamination process if a vacuum bag system is used to make the pre-press. In the assembly step, the smooth pattern allows the interlayer to tack too easily to the glass, making placement of the interlayer difficult. In the de-airing step, a smooth pattern can lead to a laminate having trapped air, and flaws in the laminate can result therefrom. It is known that interlayers having a rough surface can facilitate de-airing. Rough surface patterns can be generated by conventional methods, including use of an embossing tool to impart a reproducible pattern on the surface of the interlayer material. It is also conventional to generate a randomly irregular surface pattern by a melt-fracture process, which can provide channels by which air can escape during the lamination process.
In a typical windshield laminating process, the PVB interlayer is first subjected to a shaping step wherein the PVB interlayer is differentially stretched such that the shaped interlayer better conforms to the curvature of the vehicle for which the windshield is designed. In the shaping step, the PVB roll is unwound, and the interlayer is heated to approximately 100° C. and then passed over one or more cones which are smooth, and then chilled to approximately 10° C. for storage, and then cut into blanks slightly larger than the size of the windshield. Stresses incurred in the shaping process are partially relaxed as the blanks are conditioned at 10° C. During the shaping step, some of the pattern roughness is pressed out temporarily, but will recover according to stress relaxation kinetics well known in the art of polymer rheology.
For interlayers with surface patterns generated in a melt-fracture process, haze in a pre-press can be a problem, especially if the interlayer material is used within twelve hours of being shaped for lamination in a vacuum bag pre-pressing system. Pre-presses with less than 15% light transmission are typically rejected. Use of an embossing tool can be effective in resolving the de-airing and pre-press clarity concerns, but is more costly and more work intensive than use of a melt fracture process. An embossing process is inflexible relative to the melt fracture process, with respect to producing different patterns on the same equipment.
While use of rough patterns obtained by a melt-fracture process could improve the effectiveness of de-airing by vacuum, rough patterns generated by melt fracture require more energy to melt down in the heating step. This could render the pre-press hazier than if it had been made from a smoother interlayer. In a conventional process for making flat laminates, a glass/PVB/glass assembly is typically heated to the point where the PVB attains a temperature of abut 50-90° C. At this temperature, the entire assembly is passed through a set of nip rolls, and the nip rolls exert pressure that squeezes out the interstitial air and also seals the edges of the pre-press. Pre-presses that use conventional PVB with a roughened surface obtained by a melt fracture process tend to be hazy if Rz is above 30 micrometers.
It is desirable to obtain an interlayer material with a surface rough enough to minimize haze in a pre-press, yet maintain a desirable balance of physical properties of the interlayer, without requiring the capital investment, loss of yield, loss of flexibility, or possible contamination that can result from use of an embossing tool. Therefore it can be desirable to obtain such a rough surface without use of an embossing tool.